1. Technical Field
The present disclosure relates to high power lasers, in general, and more particularly, to systems for enabling the reliable startup of high power unstable multidisk thin disk laser (TDL) resonators.
2. Related Art
A “Thin Disk Gain Element” (TDGE), sometimes referred to as an “active mirror,” is an optical amplifier gain medium, typically but not necessarily disk-shaped, in which stimulated emission of light, i.e., “lasing,” is produced when the disk is appropriately illuminated with a pump light source and a seed laser input, resulting in an output seed laser with gain. Conventional TDGEs may be made from a ytterbium (Yb) doped yttrium aluminum garnet (YAG) crystal, i.e., Yb:YAG disks, bonded to heat sinks, such as diamond or copper. In conventional TDGE systems, the crystal of the gain medium is fixed to the heat sink with a layer of, e.g., an indium or equivalent bonding solder or adhesive. The heat sink may be liquid cooled, e.g., with water or cryogenic fluids, or with a thermoelectric (TE) cooler. For clarification purposes, a Thin-Disk Laser (TDL) represents a combination of TDGEs, a highly reflective (HR) Primary mirror, a partially reflective feedback mirror commonly referred to as an Out-Coupler (OC) and a multitude of fold mirrors (FM) that properly image and link the intra-cavity optical beam to the multiple TDGEs. Power scalable TDLs are described in detail in, e.g., U.S. Pat. Pub. No. 2008/0304534 A1 by D. Sumida et al.
One approach to achieving a solid state laser with relatively high average output power is to employ a module-scaling approach in which multiple Gain Elements (GEs) are combined within a common resonator, enabling a single-output-beam laser at power levels well beyond the capability of a laser built around a single GE. For example, welding lasers employing Yb:YAG TDGEs have demonstrated significant output power levels with outstanding optical-optical efficiency by employing resonators designed to operate in the stable regime. However these high-power welding-laser designs do not achieve near-diffraction-limited output laser beams due to the significant multi-mode content within the output laser beam. Achieving a near-diffraction-limited output laser beam requires a resonator design that achieves single transverse mode operation within the laser. Multiple-TDL laser resonator systems are described in more detail in, e.g., U.S. Pat. No. 6,987,789 to H. Brusselbach et al. and in U.S. patent application Ser. No. 12/109,634 by D. Holmes, filed Apr. 25, 2008.
Applications requiring the generation of small laser spot size at large working distances require near-diffraction-limited laser beams. The typical performance measure that characterizes a laser beam with good focusability is the beam quality factor (BQ), where a diffraction-limited beam has a BQ=1 and a BQ>1 for beams with less focusability. Military applications typically require both high power and good BQ while preserving outstanding optical-to-optical efficiencies, making these applications the most demanding. Utilization of a Negative-Branch-Imaging-Resonator (NBIR) design with a Gradient Reflectivity Mirror (GRM) Out-Coupler (OC) theoretically would enable single-transverse mode operation within the resonator. This resonator design can theoretically achieve high power through multi-module scaling and resonator fold mirrors that image TDGE to TDGE within the resonator.
However, as a practical matter, TDLs designed to achieve such objectives are complicated by temperature-dependent aberrations within the TDGEs which arise from two primary sources associated with a variation in stored energy across the pumped region of the TDGE and Amplified Spontaneous Emission (ASE) trapped within the TDGE. In addition to causing optical aberration within the TDGEs, these effects can, if not properly controlled, lead to degradation of the TDGEs via intensity-dependent effects that give rise to temperature and stress-induced degradation. These effects become more prominent with increased pump diameter and increased pump intensity, which represent the standard strategies for storing more energy within each TDGE. These effects are particularly troublesome during the start-up of the TDL, due to the dynamic nature of these effects. The dynamic effects manifest as both temporally and spatially changing aberration distributions within each TDGE, making mode-control within the resonator challenging.
The design of resonator architectures is also compromised by large variations of energy extraction within the TDLs during the start-up of the resonators. These large variations in extracted energy from the TDLs during startup act to cause disk temperature variations that lead to non-tractable disk aberrations and loss of mode control, and ultimately, prevent the resonator from establishing the desired fundamental GRM mode, and can potentially result in degradation of the TDLs due to ASE-induced damage mechanisms.
The realization of high-power multidisk TDLs with near-diffraction limited beam quality and high optical-to-optical efficiency is thus predicated on establishing and maintaining single transverse mode operation within the resonator in the presence of highly dynamic aberrations arising within the TDGEs.
A need therefore exists for methods and apparatus that enable the reliable startup of a TDL designed to produce a near-diffraction-limited output beam in the presence of dynamically changing aberrations within the TDGEs of the resonator.